As is well-known, an animal feed shortage exists worldwide. Feed which has a high protein analysis is in particularly short supply. Historically, cattle have been fed with grass hay, and the raising of such hay for winter feeding is a well-established traditional procedure.
Grass hay contains up to 20% protein if harvested in the early summer. However, there is a decline in the protein content as the summer progresses, making later cuttings of hay less valuable for cattle growth.
After July 1st, the intermountain natural grazing area of the United States produces a natural hay which has a protein content below 10% on the dry basis. When hay has a protein content below this level, the ruminant animal eating such hay does not gain weight at a satisfactory rate.
Consequently a great deal of research effort has been devoted to the establishment of diets and feeding schedules for ruminant animals which will maximize the profit to be realized in raising such animals. It was discovered early that protein-rich leguminous hay produced not only better quality meat, but also more pounds of meat per acre. This discovery has led to the use of protein-rich agricultural by-products as additives to hay to enrich the diet of the animals when hay of low protein analysis is fed.
Such agricultural products as cottonseed meal, soybean meal, peanut meal, flax seed meal, etc. are widely used along with the hay in order to upgrade the average protein analysis of the resulting feed mixture. At the present time, escalating costs of farming and ranching make it imperative that any mixed feed fed to the cattle be of lowest cost and the search for readily available, economic feeding additives of consistent quality continues.
It was discovered many years ago that it is not necessary to feed protein per se to ruminant animals in order to increase the protein content of the animal. The animal itself has the ability to convert non-protein materials into protein.
Thus, it was discovered about 1920 that inorganic nitrogen in the form of ammonium salts could be used to supplant protein in part of the diet of a ruminant animal. The ammonia content of the ammonium salts is converted by the animal into muscle tissue. Ruminants have multiple stomachs and ammonium salts release their ammonia content in the rumen, where it is converted into protein by the bacteria occurring naturally there. When the bacteria-produced protein passes into another stomach of the ruminant, it is digested just like any other feed protein.
Ammonia and ammonium salts are an inexpensive source of non-protein nitrogen utilizable in this manner. Unfortunately, there is a limit to the amount of ammonia which can be included in the diet of the animal without causing the occurrence of adverse reactions. These adverse reactions are believed to be caused by the absorption of unconverted ammonia in the digestive system of the animal.
Such absorption results in ammonolysis, a condition wherein the ammonia in the digestive system of the animal causes toxic effects and possible death. Consequently the amount of free ammonia which can be tolerated by an animal is very small. The use of ammonium salts as a source of non-protein nitrogen in mixed feeds, therefore, has not been a commercial success.
Urea is used widely today in limited amount as a non-protein nitrogen additive to mixed feeds for ruminants. The economic incentive to use urea as a source of non-protein nitrogen is great. It has its basis in the fact that urea is 46% nitrogen by weight, whereas hay analyzing 10% protein is only 1.6% nitrogen. In other words, the addition of 20 pounds of urea (1% of the dry weight) to a ton of 10% protein hay produces 2,020 pounds of hay having a protein equivalent of approximately 13%. It would require the addition of only 74.77 pounds of urea to bring the protein equivalent of the hay up to 20%.
With hay having a current market value of $75.00 per ton and urea of $165.00 per ton, the economic advantage of adding urea to hay is obvious.
However, the use of urea is generally restricted to between 1/2% and 1% of the dry weight of the mixed feed because of the toxicity which develops when urea is overfed. This toxicity is due to the hydrolysis of the urea by the enzyme urease generally present in the stomach of the ruminant animal. When urea is hydrolyzed by urease, it gives off free ammonia and the toxicity which results in the animal is in reality the earlier recognized toxicity due to free ammonia.
Because of the restriction due to ammonolysis on the use of urea, the search continued for an alternate hay additive. This search has led to the discovery that biuret, a compound which can be made from urea, can also be used by ruminant animals in the synthesis of protein.
Biuret is not readily hydrolyzed by the enzyme urease and consequently can be included in the diet of ruminants without danger of toxicity. Biuret which is unused as a symbiotic feed for the ruminant passes through the digestive tract unchanged. This makes it virtually impossible for toxic symptoms to develop when feeding biuret as a diet supplement. U.S. Pat. Nos. 2,768,895 and 2,861,886, for example, describe the use of biuret in animal feeds.
It is not necessary to use pure biuret in the supplementation of animal feeds. A technical grade product analyzing as little as 55% biuret may be used satisfactorily. The commercial standard for feed-grade biuret adopted by the U.S. Food and Drug Administration is a mixture having a minimum analysis for biuret of 55%, a maximum of 15% urea, and not more than 30% of the group consisting of cyanuric acid and other urea derivatives, by weight.
These materials result from the conversion of urea to biuret by heating the urea to a temperature above its melting point. Thereupon ammonia gas is evolved and there results a reaction mixture comprising in varying amounts the following products:
Unreacted urea PA0 Ammonium cyanate PA0 Cyanuric acid PA0 Biuret PA0 Triuret PA0 Tetrauret PA0 Higher homologs
In carrying out this pyrolytic reaction, it is not possible to achieve a yield of biuret over about 60% when operating at a temperature above the melting point of urea and at any partial pressure between 10 and 60 mm with a reaction time of from 30 minutes to 16 hours. The longer the reaction time, the greater the yield of cyanuric acid, which after a reaction time of about 12 hours is generated in ever-increasing amounts at the expense of the desired biuret product.
The production of biuret by pyrolysis of urea under elevated temperatures is well-known and a considerable art has developed as shown by the following patents showing various methods of biuret manufacture:
U.S. Pat. No. 2,145,392 to Harmon PA1 U.S. Pat. No. 2,370,065 to Olin PA1 U.S. Pat. No. 2,524,049 to Garbo PA1 U.S. Pat. No. 2,768,895 to Kamlet PA1 U.S. Pat. No. 2,861,886 to Colby PA1 U.S. Pat. No. 3,057,918 to Formaini PA1 U.S. Pat. No. 4,055,598 to Lee
All of these patented processes involve the pyrolysis of urea in the molten state or liquid phase, in a single step, which may be under vacuum, to facilitate the removal of by-product ammonia which occurs according to the following chemistry: ##STR1##
Biuret and cyanic acid also may react to form cyanuric acid as follows: ##STR2##
The formation of cyanuric acid is enhanced by elevated temperatures and reduced pressures and retarded by lower reaction temperatures.
Heretofore, purification and concentration of the biuret product has been necessary in order to produce a feed-grade biuret meeting Food and Drug Administration requirements.
This is difficult because cyanuric acid and urea cannot be separated with ease from biuret. Ordinary routines of fractional crystallization do not lead to the production of a pure, crystalline, dry biuret product. They lead rather to the formation of a wet, sticky, clay-like filter cake which contains urea in various amounts.
Also if an appreciable amount of cyanuric acid is present, the reaction mixture is viscous and sticks to the reaction and refining vessels, making it hard to handle. In addition, the cyanuric acid tends to combine with the unreacted urea compound of the reaction mixture so that it becomes virtually impossible to separate the urea by filtration, as is normally required to produce the FDA approved low-urea biuret product.
The precise mechanism by which this occurs is not known. However, urea may form an insoluble complex with cyanuric acid in the nature of a secondary valence compound. Alternatively, the urea may simply be strongly occluded by any cyanuric acid present so that in the mixed precipitate of urea, biuret, biuret homologs, and cyanuric acid, the urea is not readily removed by washing with an aqueous solvent for urea. Where a pyrolyzed urea product contains cyanuric acid in excess of 10%, and unreacted urea in excess of 35%, it is not practical to lower the urea analysis to below 15%, even by multiple recrystallizations.
The disclosure of Lee U.S. Pat. No. 4,005,598 includes a good summarization of various prior art processes for pyrolyzing urea to produce reaction products which are principally biuret. The contribution to the art offered by the Lee patent disclosure is that of a process for producing biuret wherein the extent of pyrolytic conversion of urea to biuret is increased, with less than normal amounts of cyanuric acid, triuret and other biuret homolog products being formed, the reaction involving a starting material made up of a low urea, high biuret seed material mixed with a high urea, low biuret feedstock material, which mixture has a total urea content not exceeding 20% by weight, such mixture being heated to a range of from about 100.degree. C. to about 150.degree. C. and preferably from about 115.degree. C. to about 125.degree. C. while dispersed as a slurry in an alkane series hydrocarbon carrier having from 8 to 12 carbon atoms and a boiling point at or above the reaction temperature. In the Lee process, heating of the mixture of low urea seed material and high urea feedstock material proceeds with the mixture in slurry form in the liquid hydrocarbon carrier, with by-product ammonia being removed by evolution of hydrocarbon carrier vapor. This process, although assertedly achieving the objective of permitting the reaction to proceed at relatively low temperature in order to minimize the formation of cyanuric acid and other undesired auto-condensation products, has manifest practical disadvantages in that it requires an essentially liquid phase reaction environment and the toleration of potentially dangerous reaction conditions since the reaction is carried out in the presence of a heated liquid hydrocarbon carrier which inherently presents an explosion or fire hazard.
The Lee process is severely limited in production capacity in that the urea content of the reaction mass is limited to a maximum of 20%. This limitation in the Lee process is dictated by the fact that the Lee reaction product will gum up and not remain a slurry in the liquid medium and in the processing equipment if the urea content exceeds 20%. In contrast, applicant's dry, solid state pyrolyzation process is operable with up to about 60% urea content by weight in the heated reaction mass, which correspondingly increases production capacity insofar as the amount of urea which can be converted to a reaction product suitable for use as feed grade biuret in a given amount of time.
A further disadvantage of the Lee process is that its end product is oil contaminated in the sense of the product retaining some residual alkane hydrocarbon, which detracts from its palatability when used as animal feed. There is also the possibility that residual pyrolyzed hydrocarbon may render the product unsuitable for animal consumption and indirectly human consumption in view of possible carcinogenic risk. Furthermore, a residual oil content in the end product in this type of process may interfere or complicate use of the product or portions thereof in water solution. As a related consideration, the use of an oily, hydrophilic carrier as the reaction medium can complicate evolved ammonia by-product recovery in aqueous solution since the by-product is then recovered in what amounts to an oil-in-water emulsion.